In previous shipboard radar systems, targets at long range have been difficult to detect visually on plan position indicator (PPI) displays. For a given range scale setting, the echoes from targets at long range are much weaker than those at short range. Accordingly, without any processing compensation, the video signals transferred to the PPI display for long range targets do not produce sufficient electron beam energy to illuminate the phosphor so as to compare with the intensity of short range targets. Furthermore, if the PPI display is presented with a display signal having discrete levels resulting from threshold detection, identical targets at different ranges will be displayed for different time durations because the longer range echoes are weaker and remain above the threshold for a shorter period of time. The long range targets appear only as thin arcs and are thus difficult to detect visually.
Sensitivity time control (STC) has been used in the prior art to reduce the receiver gain for short ranges so that target echoes can be detected in the presence of strong sea clutter. Although the primary purpose of STC has been to correct for the effect of strong sea clutter illuminating the entire region near the center of a PPI display, it has also been useful in compensating for echos at different ranges having different amplitudes. However, because of the limited dynamic range of receivers, STC is not an effective way of compensating for echoes of varying amplitudes at long ranges.
In prior art Marine radars, the transmitted pulse widths have been increased for the long range settings. Even though these longer pulses sacrifice the greater definition for shorter range targets, they may be required to obtain acceptable signal-to-noise ratios for long range targets. Although longer transmitted pulse widths substantially improve the PPI display for long range targets, they are not an effective solution to improving the overall display because they equally increase the display duration of short range targets.
Another method of improving the visual display of long range targets is described in U.S. Pat. No. 2,472,209, wherein the pulses of the display signal are increased in time duration to increase the detectability of long range targets. This solution to the visual display of long range targets, however, distorts the visual display of short range targets which prior to processing possessed sufficient intensity for proper display.